Start Blender with popen in windows 7
It's because of the escape character "\" and how python handles it. Windows paths on python scripts are a nightmare. So you have to escape the backslash.

Try :

blendFile = "D:\\_Blender_Files\\atrium.blend"
blenderPath = r"C:\\Program Files\\Blender Foundation\\blender-2.71-windows64\\blender.exe"
subprocess.Popen([blenderPath, blendFile], stdout=subprocess.PIPE)


or with simple slashes (python handles it well) :

blendFile = "D:/_Blender_Files/atrium.blend"